Bring the water to the boil with the salt. Stir in the polenta, reduce the heat, simmer over a low heat for approx. 10 mins. (or as per the packet instructions), stirring frequently to form a thick paste, stir in the butter. Transfer the warm polenta to a flat tray rinsed with cold water (if the tray is small, the polenta will be thicker and can be cut in half crosswise later).
Leave the polenta to cool (overnight if necessary).
Cut the polenta into strips approx. 2 cm wide, place on a baking tray lined with baking paper, brush with oil, sprinkle with fleur de sel and rosemary.
Approx. 15 mins. in the centre of an oven preheated to 200 °C until the fries are crispy. Move the tray to the top third for the final few minutes if necessary.